The invention relates also to an assembly comprising such a filter and its implanting device.
In order to implant these filters in the body of a patient, the practitioner generally uses the intravenous route, either exposing the vessel or using an introduction technique referred to as the percutaneous technique.
The percutaneous technique is the less aggressive because a device of the type used for introducing a catheter can be used in order to introduce the filter via the jugular or femoral vein. When an incision has been made in the skin, the filter is introduced into the vein concerned and is then guided as far as the inferior vena cava where it is "expelled" from the device. It expands in the vein, thereby preventing the migration of clots moving towards the heart and the pulmonary artery, and avoiding embolism.
Depending on the risks involved, two major categories of filters are available today.
First of all, in the face of a serious risk of embolism, filters referred to as "definitive" filters are proposed, that is to say, filters intended for permanent implantation.
Such filters are described, for example, in patents U.S. Pat. No. 3,540,431, U.S. Pat. No. 4,688,553 or in FR-A-2 689 388.
In those two typical cases, the filter is made up of filiform elements (round filaments or the like) arranged so as to define a corolla, the axis of which must advantageously be orientated substantially along the axis of the vessel, in order to obtain the best clot-retaining effect, without disturbing to an excessive extent the circulation of blood in respect of which the filter is permeable.
Having been implanted definitively, those filters have to be secured to the vein and, to that end, they often comprise hooks which anchor themselves in the wall of the vessel at the moment when the filter is released and expands radially (often by itself, a number of filters being "self-expansible").
At present, the implanting device for those filters comprises a sleeve which extends from outside the patient's body and in which the filter is retained in the radially restrained state, and a maneuvering rod which is mounted to slide in the sleeve and which permits, at the appropriate moment, the expulsion of the filter out of that sleeve, which is then withdrawn with the rod.
In some cases, however, the risk of pulmonary embolism does not necessitate permanent in situ filtration treatment. In that case, filters referred to as "temporary" filters have already been proposed (for example described in FR-A-2 694 687 or FR-A-2 657 261), that is to say, filters having a structure such that they can be withdrawn after having been implanted.
Usually, they are medical instruments having an external sleeve in which slides a maneuvering rod, at one end of which a filtering means is secured in a definitive manner and may, in particular, again be in the form of a filiform structure developing naturally in the shape of a conical corolla which is radially restrained in the sleeve as long as the filter is not in operation, the releasing of the filter still being effected by relative axial displacement of the sleeve and the rod, which enables the filtration corolla to open out. Insofar as the filtering means do not have an anchoring hook and are held by the rod, they can be withdrawn by the venous access route.
These temporary or definitive filters have the disadvantage of being selective, that is to say, if the practitioner first of all judges it necessary to use a temporary filter and then later judges that a definitive filter is required, it is absolutely necessary for him to withdraw the first and then implant the second. Conversely, if he has implanted a definitive filter straight away, it is too late to think of using a temporary filter.
In view of the, sometimes great, difficulty which the practitioner has in evaluating straight away the degree of risk of embolism, EP-A-0 348 295 has proposed a filtration assembly for temporary and/or definitive use which comprises, on the one hand, a filter having flexible legs defining two head-to-tail corollas (one for filtration and one for retaining the filter definitively implanted in the vessel) and, on the other hand, a double catheter for retaining the filter in temporary implantation. The double catheter comprises an external catheter which, when the assembly is used temporarily, contains the filter-retaining corolla which is then in a radially restrained state (the filtration corolla being expanded in the vessel) and an internal catheter which is mounted to slide in the external catheter for the complete expulsion of the filter from the external catheter (on passing from temporary use to definitive use of the filter). In the case of temporary use, the filter is retained connected to the double catheter simply by the friction of the legs of its retaining corolla against the wall of the external catheter.
In the case of definitive use, the filter is completely expanded in the vessel, with its retaining corolla supported against the wall of the vessel in order to secure the filter there, the double catheter normally being withdrawn from the patient's body.
In order to position the assembly, an implanting apparatus is used which comprises an implanting sleeve through which the double catheter carrying the filter is made to slide, the double catheter forming a pusher for the filter in order to direct it through the sleeve as far as the implantation area.
In practice, this assembly has not always proved to be completely reliable and has been found to be complicated and tricky to manipulate for the implantation of the filter. It has been noted in particular that:
three coaxial tubes (sleeve and double catheter) are necessary for the correct implantation of the filter, which increases the bulk (cross-section) of the system and complicates its introduction into sinuous or narrow channels;
any accidental movement of the internal catheter relative to the external catheter would cause undesirable displacement of the filter inside the external catheter (which could weaken the support of the filter) and even an expulsion of the filter from the sleeve;
the internal catheter acts on the filter at the proximal end of the latter and apart from the head, which does not always make its displacement easy;
in addition, the internal catheter displaces the filter only in the direction of its expulsion from the external catheter, which does not always permit a control of the displacement of the filter, especially for satisfactory positioning in the vessel.
In order to control somewhat the ejection of a definitive filter from its implanting sleeve, it has already been proposed, in FR-A-26 45 028, to equip a conventional system for the implantation of definitive filters with means for the releasable connection of the pusher-rod to the proximal end of the filter, that rod then having a recessed distal end for receiving the head of the filter and gripping it for as long as it is tightly arranged inside the sleeve.
To the applicant's knowledge, there has never yet been proposed an assembly for temporary and/or definitive filtration which has a simple structure, is reliable and comprises releasable connection means provided on the pusher-rod and on the filter. Even if it had been decided to use the relatively simple system provided with releasable connection means of FR-A-26 45 028 for implanting the filter for temporary and/or definitive use of EP-A-0 348 295, the mounting solution naturally adopted, bearing in mind the teaching of those documents, would have been to clamp the proximal end of the legs of the corolla for retaining the filter in the recessed distal end of the rod, such a method of mounting not really providing any solution to the problems of control and facility of displacement of the filter by the pusher-stem.